The Indian Ocean Dipole (IOD) consists of opposite signed sea surface temperature (SST) anomalies in the western and eastern tropical Indian Ocean, with IOD positive when the western SST anomaly is positive, the eastern is negative, and vice versa for IOD negative. As the IOD often appears during active ENSO, isolating its impact on the general circulation in observations is challenging. To isolate the IOD impact, we only consider IOD during ENSO neutral conditions.
We examined the effects of both phases of the IOD on the atmospheric general circulation using observations, the CESM Large Ensemble (historical) and perturbation experiments in CAM5, which is the atmospheric component of the CESM. We find two mechanisms by which the IOD can affect the large-scale circulation in South America. A perturbation in the Walker circulation during the positive phase of the IOD leads to increased convection over the Amazon basin and wet anomalies in the region. Meanwhile, a subtropical Rossby wave train forced by anomalous convection over the Indian Ocean arches across the Pacific and eventually reaches South America in both phases of the IOD. During the negative IOD, the anomalies related to this wave train strengthen the moisture transport by the South American low-level jet to the La Plata basin, generating wet anomalies in the region. During the positive IOD, the wave train strengthens the South Atlantic Subtropical High and weakens the South Atlantic Low-Level Jet, generating dry anomalies in the area where the South Atlantic Convergence Zone is commonly observed.